1. Field of the Invention
The present invention relates to a polishing slurry, particularly, a slurry for chemical mechanical polishing (hereinafter, referred to as ‘CMP’), which is used in a chemical mechanical polishing process for flattening a semiconductor laminate. More particularly, the present invention pertains to a method of producing a slurry which has high removal selectivity to a nitride layer used as a barrier film in a shallow trench isolation CMP process needed to fabricate ultra highly integrated semiconductors of 256 mega D-RAM or more (Design rule of 0.13 μm or less) and which decreases the occurrence of scratches on a flattened surface, and a method of polishing a substrate using the same.
2. Description of the Related Art
Chemical mechanical polishing (CMP) is a semiconductor processing technology in which a mechanical process using polishing particles between a pressed wafer and a polishing pad and chemical etching using a slurry are simultaneously conducted, and has been an essential process of global planarization technology in the production of submicron-scaled semiconductor chips since IBM Co., Ltd. in the USA developed it at the end of the 1980's.
The types of slurry are roughly classified into a slurry for oxide, a slurry for metal, and a slurry for poly-silicon according to the type of object to be polished. The slurry for oxide is used to polish an interlayer insulating film and a silicon oxide (SiO2) layer employed in an STI (shallow trench isolation) process, and roughly comprises polishing particles, deionized water, a pH stabilizer, and a surfactant. The polishing particles function to mechanically polish the surface of the object by means of pressure from a polishing machine, and are exemplified by silica (SiO2), ceria (CeO2), and alumina (Al2O3).
Particularly, ceria slurry is frequently used to polish the silicon oxide layer during the STI process, and in this case, a silicon nitride layer is mainly used as a polishing stopper layer. Hence, an additive is added to the ceria slurry to reduce the removal speed of the nitride layer so as to improve the polishing speed selectivity of the oxide layer to the nitride layer. However, the use of the additive is disadvantageous in that the removal speed of the oxide layer, as well as the removal speed of the nitride layer, is reduced. Furthermore, the polishing agent of the ceria slurry typically has particles larger than those of the silica slurry, and therefore scratches the surface of the wafer.
However, if polishing speed selectivity of the oxide layer to the nitride layer is low, a dishing phenomenon, in which an excessive volume of the oxide layer is removed, occurs due to the loss of adjacent nitride layer patterns. Thus, it is impossible to achieve uniform surface flattening.
Accordingly, the slurry for STI CMP requires high selectivity and polishing speed, dispersion and micro-scratch stabilities, and narrow and uniform particle size distribution. Additionally, the number of large particles having the size of 1 μM or more must exist within a predetermined range.
With respect to conventional technology of producing the slurry for STI CMP, U.S. Pat. Nos. 6,221,118 and 6,343,976, granted to Hitachi Inc., disclose a method of synthesizing ceria particles and a method of producing a slurry having high selectivity using the same. These patents describe characteristics of particles required in the slurry for STI CMP, the type of additives containing polymer, and the production method using them in very critical and wide ranges. Particularly, the patents suggest wide ranges of an average grain size, an average primary particle size, and an average secondary particle size. Particularly, they mention a change of the grain size depending on calcination temperature, and scratches corresponding to this. In another conventional technology, U.S. Pat. No. 6,420,269, granted to Hitachi Inc., discloses a method of synthesizing various ceria particles and a method of producing a slurry having high selectivity using the same. Meanwhile, U.S. Pat. No. 6,615,499, granted to Hitachi Inc., discloses a change of ratios of peak intensities in a predetermined range of X-rays, which depends on a rate of temperature increase in a calcination process, and a change of a removal rate according to this. Furthermore, in the prior arts, U.S. Pat. Nos. 6,436,835, 6,299,659, 6,478,836, 6,410,444, and 6,387,139, which have been made by Showa Denko Co. Ltd. in Japan, disclose a method of synthesizing ceria particles and a method of producing a slurry having high selectivity using the same. These patents mostly describe the types of additives added to the slurry, effects due to them, and a coupling agent.
However, the above prior arts disclose only the average particle size of the polishing particles constituting the polishing slurry and the range thereof, but lack details on how to disperse the particles. If it is considered that a dispersion state and a particle size distribution of ceria powder are very remarkably dependent on the degree of improvement of properties and dispersion stability thereof, and that, therefore, the degree of improvement significantly affects the number of micro-scratches, it becomes very important to find process conditions capable of assuring optimum dispersion stability using properties and dispersion of the ceria powder, which depend on calcination conditions. Furthermore, it is very important to find process conditions capable of assuring optimum dispersion stability by mixing with an appropriate amount of dispersing agent and using dispersing devices, and to provide a slurry formed through the resultant process.